High electron mobility transistor (HEMT) device

ABSTRACT

A high electron mobility transistor (HEMT) device with epitaxial layers that provide an electron Hall mobility of 1080±5% centimeters squared per volt-second (cm 2 /V·s) at room temperature for a charge density of 3.18×10 13 /cm 2  and method of making the HEMT device is disclosed. The epitaxial layers include a channel layer made of gallium nitride (GaN), a first spacer layer made of aluminum nitride (AlN) that resides over the channel layer, a first spacer layer made of Al X Ga (1-X) N that resides over the first spacer layer, and a first barrier layer made of Sc y Al z Ga (1-y-z) N that resides over the second spacer layer. In at least one embodiment, a second barrier layer made of Al X Ga (1-X) N is disposed over the first barrier layer.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 15/299,571, filed Oct. 21, 2016, now U.S. Pat. No. 10,636,881,which claims the benefit of provisional patent application Ser. No.62/320,910, filed Apr. 11, 2016, the disclosures of which are herebyincorporated herein by reference in their entireties.

FIELD OF THE DISCLOSURE

The present disclosure relates to a high electron mobility (HEMT) deviceand, in particular, to reducing lattice stress between semiconductorlayers while maintaining a high sheet charge density within the HEMTdevice.

BACKGROUND

In order to manufacture reliable gallium nitride (GaN) based highelectron mobility transistor (HEMT) devices and circuits, the amount ofresidual stress in a device's epitaxial structure must be controlledsuch that the effects of inverse piezoelectric stresses and/or stressenhanced corrosion of barrier layers is mitigated. Several methods ofstress control have been investigated. Some of these methods includereplacing barrier layers with indium aluminum nitride (InAlN) or indiumgallium aluminum nitride (InGaAlN) lattice matched alloys. Other methodsfocus on lattice engineering the buffer layer with low aluminumconcentration aluminum gallium nitride (AlGaN), AlGaN/GaN superlatticesor dilute boron gallium nitride (BGaN) alloys. However, such approachesin reducing lattice stress have certain limitations.

Another issue pertains to maintaining high sheet charge density within aHEMT device while providing lattice stress relief. The high sheet chargedensity and high breakdown field of GaN based field effect transistors(FETs) enables significantly higher power operation compared totraditional group III-V HFETs. Commercially available GaN FETs have asheet charge density <1×10¹³ cm⁻². However, the highest charge densityavailable for a GaN based materials system is greater than 5×10¹³ cm⁻²,which has been demonstrated with an AlN/GaN HFET. In such a system, theAlN is fully strained. However, the AlN barrier layer can readily formmicro cracks if the thickness of the AlN barrier layer is beyond thecritical thickness for lattice relaxation. In fact, even with an AlNthickness of approximately 4 nm, micro cracks would develop over time.Though the high sheet charge in an AlN/GaN system is attractive, thelimitation of the strain makes a FET or HEMT device applicationimpractical. To reduce strain energy, the thickness of the AlN barrierlayer is reduced, which in turn dramatically reduces the sheet chargedensity. As such, an AlN system is far less attractive in comparison toa conventional AlGaN/GaN system.

One approach to minimizing total strain is the use of a lattice matchedInAlN/GaN system. The sheet charge in a lattice matched condition isapproximately 2.6×10¹³ cm⁻². However, the poor crystal quality of InAlN,due mainly to phase segregation, limits applications for FETs and HEMTs.Furthermore, in addition to a very narrow growth window for closelylattice matched InAlN/GaN, sheet charge density decreases dramaticallywhen the barrier is slightly compressively strained. As such, a needremains for providing lattice stress relief while maintaining high sheetcharge density within a HEMT device.

SUMMARY

A high electron mobility transistor (HEMT) device with epitaxial layersthat include a gallium nitride (GaN) layer and an aluminum (Al) basedlayer having an interface with the GaN layer is disclosed. The Al basedlayer includes Al and an alloying element that is selected from GroupIIIB transition metals of the periodic table of elements. The epitaxiallayers are disposed over the substrate. A gate contact, a drain contact,and a source contact are disposed on a surface of the epitaxial layerssuch that the source contact and the drain contact are spaced apart fromthe gate contact and each other. The alloying element relieves latticestress between the GaN layer and the Al based layer while maintaining ahigh sheet charge density within the HEMT device. Further still, a HEMTdevice with epitaxial layers that provide an electron Hall mobility of1080±5% centimeters squared per volt-second (cm²/V·s) at roomtemperature for a charge density of 3.18×10¹³/cm² and method of makingthe HEMT device are disclosed. The epitaxial layers include a channellayer made of GaN, a first spacer layer made of aluminum nitride (AlN)that resides over the channel layer, a first spacer layer made ofAl_(X)Ga_((1-X))N that resides over the first spacer layer, and a firstbarrier layer made of Sc_(y)Al_(z)Ga_((1-y-z))N that resides over thesecond spacer layer. In at least one embodiment, a second barrier layermade of Al_(X)Ga_((1-X))N is disposed over the first barrier layer.

Those skilled in the art will appreciate the scope of the presentdisclosure and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure, andtogether with the description serve to explain the principles of thedisclosure.

FIG. 1 is a cross-sectional diagram of a prior art gallium nitride (GaN)high electron mobility transistor (HEMT) that can benefit from theteachings of the present disclosure.

FIG. 2 is a cross-sectional diagram of a GaN HEMT of the presentdisclosure.

FIG. 3 is a cross-sectional diagram of a double heterostructure GaN HEMTof the present disclosure.

FIG. 4 is a graph of calculated in plane lattice constant of scandiumaluminum nitride (Sc_(X)Al_(1-X)N) as a function of Sc concentration insolid phase.

FIG. 5 is a graph comparing the bandgap of indium aluminum nitride(InAlN) and scandium aluminum nitride (ScAlN) as a function of Sc and Inconcentrations.

FIG. 6 is a cross-sectional diagram of a ScAlN/GaN structure.

FIG. 7 is a graph comparing calculated sheet charge density as afunction of Sc and In concentrations.

FIG. 8 is a cross-sectional diagram of a GaN HEMT having improvedelectron Hall mobility in accordance with the present disclosure.

FIG. 9 is a cross-sectional diagram of a GaN HEMT having improvedelectron Hall mobility and improved breakdown and/or reliability inaccordance with the present disclosure.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the embodiments andillustrate the best mode of practicing the embodiments. Upon reading thefollowing description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the disclosureand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element such as a layer, region, orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present.Likewise, it will be understood that when an element such as a layer,region, or substrate is referred to as being “over” or extending “over”another element, it can be directly over or extend directly over theother element or intervening elements may also be present. In contrast,when an element is referred to as being “directly over” or extending“directly over” another element, there are no intervening elementspresent. It will also be understood that when an element is referred toas being “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer, or region to another element, layer, or region asillustrated in the Figures. It will be understood that these terms andthose discussed above are intended to encompass different orientationsof the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including” when used herein specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.For this disclosure room temperature is defined to be between 20° C. and25° C.

FIG. 1 is a cross-sectional diagram of a prior art gallium nitride (GaN)high electron mobility transistor (HEMT) 10 that can benefit from theteachings of the present disclosure. The prior art GaN HEMT 10 hasepitaxial layers 12 formed over a substrate 14. In this exemplary case,the substrate 14 is made of silicon carbide (SiC). Other substratematerials such as sapphire and silicon are also available. The epitaxiallayers 12 include a nucleation layer 16 made of aluminum nitride (AlN),a buffer layer 18 made of GaN, a barrier layer 20 made of AlGaN, a caplayer 22 made of GaN, and an optional passivation layer 24 made ofsilicon nitride (SiN). A source contact 26, a gate contact 28, and adrain contact 30 are disposed on a surface of the epitaxial layers 12.The buffer layer 18 is approximately 2 μm thick, and the barrier layer20 is 10 to 30 nm thick.

FIG. 2 is a cross-sectional diagram of a GaN based HEMT device 32 of thepresent disclosure. The present disclosure provides lattice stressrelief while maintaining high sheet charge density within the HEMTdevice 32. Generally, the HEMT device 32 has epitaxial layers 34 thatinclude a GaN layer 36 that is equivalent to the buffer layer 18depicted in FIG. 1. Also included is an aluminum (Al) based layer 38having an interface 40 with the GaN layer 36. In this case the Al basedlayer 38 serves the same function as the barrier layer 20 depicted inFIG. 1. The Al based layer 38 comprises Al and an alloying element 42that is selected from Group IIIB transition metals of the periodic tableof elements. In an exemplary embodiment, the alloying element 42 isscandium (Sc), which is used to form scandium aluminum gallium nitride(ScAlGaN).

An Sc concentration of 18% in Sc_(X)Al_((1-x)) is predicted to be alattice match for GaN. However, a concentration in the range of 1% to 5%of the alloying element 42 within the Al based layer 38 provides arelatively higher sheet charge density while providing the samereliability as modern HEMT devices. In other embodiments, theconcentration of the alloying element in a buffer layer such as Al basedlayer 38 is in the range of 0.5% to 5%. Some modern HEMT devices includean AlGaN barrier layer having up to 23% Al with a 20 nm thickness. Amuch higher sheet charge density could be achieved by increasing theconcentration of Al to 50% while maintaining the 20 nm thickness.However, increasing the Al concentration to 50% would reduce thereliability of such a HEMT device to an impractical level. In contrast,the present disclosure's addition of Sc as the alloying element at a 5%concentration within the Al based layer 38 provides a sheet chargedensity of 2.2×10¹³ cm⁻², which is practically the same sheet chargedensity expected by increasing the Al to 50%. In the embodiment of FIG.2, the Al based layer 38 is made of Sc_(0.05)Al_(0.45)Ga_(0.50)N. Atwo-dimensional electron gas (2DEG) having the sheet charge density of2.2×10¹³ cm⁻² is formed at the interface 40.

It is to be understood that other alloying elements and thicknesses forthe Al based layer 38 are available depending on the performance goalsfor the HEMT device 32. For example, other alloying elements such asyttrium (Y) having an oxidation state of +3 are available. In someembodiments, the yttrium is in the form of yttrium aluminum nitride(YAlN) or yttrium aluminum gallium nitride (YAlGaN).

FIG. 3 is a cross-sectional diagram of a double heterostructure GaN HEMT44 having epitaxial layers 46 of the present disclosure. In thisembodiment, Sc is employed as the alloying element 42 with aconcentration of in the range of 1% to 5% within the Al based layer 38for use as a buffer and/or a back barrier layer. The lattice constantfor the Al based layer 38 used as a buffer layer is adjustable duringmanufacture to increase Al concentration without sacrificingreliability. As such, in this particular embodiment, the Al based layer38 has increased Al content that makes the Al based layer 38particularly attractive for forming the double heterostructure HEMTdevice 44 that further includes an aluminum gallium (AlGaN) layer 48that is over the GaN layer 36. The double heterostructure nature of thedouble heterostructure HEMT device 44 improves carrier confinement toreduce short channel effects in scaled devices.

FIG. 4 is a graph of calculated in plane lattice constant (a) ofscandium aluminum nitride (ScAlN) as a function of Sc concentration insolid phase. The graph shows a lattice match condition between ScAlN andGaN when the concentration of Sc is approximately 18%. For the purposeof this disclosure a lattice match condition is defined as being within1% of the lattice constant of GaN. Therefore, the concentration of Sc isin the range of 17% to 19% to achieve a lattice match with GaN. In otherembodiments, the alloying element 42 has a concentration within the Albased layer that is between 5% and 17%.

In some embodiments, ScAlN/GaN and ScAlGaN/GaN interfaces are latticematched by adding an appropriate concentration of an alloying element toan Al based layer, which in this example is either ScAlN/GaN orScAlGaN/GaN. ScAlN is attractive for lattice matching because ScAlN isrelatively easy to deposit, and ScAlN exhibits no phase segregation upto an Sc concentration of 30%. Moreover, ScAlN and ScAlGaN matchedlattices allow HEMT devices fabricated in accordance with the presentdisclosure to have sheet charge densities set by varying alloycompositions. For example, a lattice matched Sc_(0.18)Al_(0.72)N/GaN hasa calculated sheet charge density that is about 5.1×10¹³ cm⁻², which isonly a fraction less than an AlN/GaN heterojunction structure. It isalso observed that sheet charge density only decreases by 2% for Scconcentrations that are greater than 18% when the lattice is within 1%of being matched.

FIG. 5 is a graph comparing the bandgap of indium aluminum nitride(InAlN) and scandium aluminum nitride (ScAlN) as a function of Sc and Inconcentrations. Note that for a lattice match condition, ScAlN has alarger bandgap compared to the bandgap of indium aluminum nitride(InAlN).

FIG. 6 is a cross-sectional diagram of a ScAlN/GaN structure. In thisexemplary embodiment, the interface 40 is lattice matched usingScAlN/GaN, and FIG. 7 is a graph comparing calculated sheet chargedensity as a function of Sc and In concentrations. The graph of FIG. 7depicts the lattice match condition for each of the alloyingcompositions ScAlN/GaN and InAlN. Notice that a lattice matchedScAlN/GaN HEMT device has a sheet charge density of 5.1×10¹³ cm⁻², whichis close to 5.6×10¹³ cm⁻² maximum for a lattice matched InAlN/GaN HEMTdevice. In other embodiments, a sheet charge on the order of 2.0×10¹³cm⁻² is useful for high breakdown voltage devices.

FIG. 8 is a cross-sectional diagram of a GaN HEMT device 50 havingimproved electron Hall mobility in accordance with the presentdisclosure. The GaN HEMT device 50 has epitaxial layers 52 formed over asubstrate 54. In this exemplary case, the substrate 54 is made ofsilicon carbide (SiC). Other substrate materials such as sapphire,silicon, and thermally conductive polymers are also available. Theepitaxial layers 52 include a buffer layer 56 made of GaN, a channellayer 58 made of GaN, a first spacer layer 60 made of AlN, a secondspacer layer 62 made of Al_(X)Ga_((1-X))N (0<x<0.5), and a barrier layer64 made of Sc_(y)Al_(z)Ga_((1-y-z))N. A source contact 66, a gatecontact 68, and a drain contact 70 are disposed on a surface of theepitaxial layers 52.

At least one advantage of combining the first spacer layer 60 made ofAlN and the second spacer layer 62 made of Al_(X)Ga_((1-X))N is that theelectron Hall mobility is improved from about 800 centimeters squaredper volt-second (cm²/V·s) at room temperature for the previousembodiments to 1080 cm²/V·s ±5% at room temperature for the embodimentof FIG. 8 for the same charge density of 3.18×10¹³/cm². This relativelygreatly improved electron Hall mobility provides a sheet resistancewithin the channel layer 58 of no more than 195 Ohms/square.

Another advantage of disposing the first spacer layer 60 made of AlNbetween the channel layer 58 made of GaN and the second spacer layer 62made of Al_(X)Ga_((1-X))N is that disposing the binary material AlNadjacent to the channel layer 58 made of GaN reduces carrier scatteringrelative to the ternary material Al_(X)Ga_((1-X))N that makes up thesecond spacer layer 62. In this embodiment the carrier is electrons, andin at least some embodiments the first spacer layer 60 made of AlN isdisposed directly onto the channel layer 58 with no intervening layers.

Thicknesses of the first spacer layer 60 and the second spacer layer 62also contribute to improved performance of the GaN HEMT 50. In theembodiment of FIG. 8, the thickness of the first spacer layer 60 isbetween 5 Angstroms (Å) and 15 Å, and the thickness of the second spacerlayer is between 10 Å and 50 Å. The thickness of the first spacer layer60 is limited by strain on the AlN material, so the second spacer layer62 requires a relatively increased thickness to protect the AlN fromoxidation. The combined thickness of the first spacer layer 60 and thesecond spacer layer 62 is large enough to prevent carrier scatteringfrom the barrier layer 64 made of Sc_(y)Al_(z)Ga_((1-y-z))N. A thicknessof the barrier layer 64 made of Sc_(y)Al_(z)Ga_((1-y-z))N is between 30Å and 300 Å.

FIG. 9 is a cross-sectional diagram of a GaN HEMT device 72 havingimproved electron Hall mobility and improved breakdown and/orreliability in accordance with the present disclosure. The GaN HEMTdevice 72 has epitaxial layers 74 formed over a substrate 54. Adifference between the epitaxial layers 74 and the epitaxial layers 52of the previous embodiment of FIG. 8 is an addition of a second barrierlayer 76 made of Al_(X)Ga_((1-X))N (0<x<0.5). In other embodiments, acap layer such cap layer 22 of FIG. 2 can be disposed over the secondbarrier layer 76. In other embodiments, the barrier layer 64 made ofSc_(y)Al_(z)Ga_((1-y-z))N is interchanged in position within theepitaxial layers 74 with the second barrier lay 76 made ofAl_(X)Ga_((1-X))N, so that the second barrier layer 76 is adjacent tothe second spacer layer 62.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present disclosure. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A high electron mobility transistor (HEMT) devicecomprising: a substrate; epitaxial layers over the substrate andcomprising; a channel layer made of gallium nitride (GaN); a firstspacer layer made of aluminum nitride (AlN) that resides over thechannel layer; a second spacer layer made of Al_(X)Ga_((1-X))N thatresides over the first spacer layer; and a first barrier layer made ofSc_(y)Al_(z)Ga_((1-y-z))N that resides over the second spacer layer,wherein y is at least 0.005; and a gate contact disposed on a surface ofthe epitaxial layers; a source contact disposed on the surface of theepitaxial layers; and a drain contact disposed on the surface of theepitaxial layers, wherein the source contact and the drain contact arespaced apart from the gate contact and each other.
 2. The HEMT device ofclaim 1 having an electron Hall mobility of 1080±5% centimeters squaredper volt-second (cm²/V·s) at room temperature for a charge density of3.18×10¹³/cm².
 3. The HEMT device of claim 2 wherein a sheet resistancewithin the channel layer is no more than 195 Ohms/square.
 4. The HEMTdevice of claim 1 wherein the first spacer layer has a thickness between5 Angstoms (Å) and 15 Å.
 5. The HEMT device of claim 1 wherein thesecond spacer layer has a thickness between 10 Å and 50 Å.
 6. The HEMTdevice of claim 1 wherein the first spacer layer has a thickness between5 Å and 15 Å, and the second spacer layer has a thickness between 10 Åand 50 Å.
 7. The HEMT device of claim 5 wherein a combined thickness ofthe first spacer layer and the second spacer layer prevents carrierscattering from the first barrier layer made ofSc_(y)Al_(z)Ga_((1-y-z))N.
 8. The HEMT device of claim 1 wherein the afirst barrier layer made of Sc_(y)Al_(z)Ga_((1-y-z))N is between 30 Åand 300 Å thick and wherein y is between 0.005 and 0.3.
 9. The HEMTdevice of claim 1 further including a second barrier layer, wherein thesecond barrier layer is made of Al_(X)Ga_((1-X))N resides over the firstbarrier layer and is between 10 Å and 100 Å thick.
 10. The HEMT deviceof claim 1 further including a buffer layer made of GaN that residesbetween the substrate and the channel layer.
 11. A method of making aHEMT device comprising: providing a substrate; disposing over thesubstrate, epitaxial layers comprising; a channel layer made of galliumnitride (GaN); a first spacer layer made of aluminum nitride (AlN) thatresides over the channel layer; a second spacer layer made ofAl_(X)Ga_((1-X))N that resides over the first spacer layer; and a firstbarrier layer made of Sc_(y)Al_(z)Ga_((1-y-z))N that resides over thesecond spacer layer, wherein y is at least 0.005; and disposing a gatecontact on a surface of the epitaxial layers; disposing a source contacton the surface of the epitaxial layers; and disposing a drain contact onthe surface of the epitaxial layers, wherein the source contact and thedrain contact are spaced apart from the gate contact and each other. 12.The method of making a HEMT device of claim 11 wherein the HEMT devicehas an electron Hall mobility of 1080±5% centimeters squared pervolt-second (cm²/V·s) at room temperature for a charge density of3.18×10¹³/cm².
 13. The method of making the HEMT device of claim 12wherein a sheet resistance within the channel layer is no more than 195Ohms/square.
 14. The method of making the HEMT device of claim 11wherein the first spacer layer has a thickness between 5 Å and 15 Å. 15.The method of making the HEMT device of claim 11 wherein the secondspacer layer has a thickness between 10 Å and 50 Å.
 16. The method ofmaking the HEMT device of claim 11 wherein the first spacer layer has athickness between 5 Å and 15 Å, and the second spacer layer has athickness between 10 Å and 50 Å.
 17. The method of making the HEMTdevice of claim 15 wherein a combined thickness of the first spacerlayer and the second spacer layer prevents carrier scattering from thefirst barrier layer made of Sc_(y)Al_(z)Ga_((1-y-z))N.
 18. The method ofmaking the HEMT device of claim 11 wherein the a first barrier layermade of Sc_(y)Al_(z)Ga_((1-y-z))N is between 30 Å and 300 Å thick andwherein y is between 0.005 and 0.3.
 19. The method of making the HEMTdevice of claim 11 further including disposing a second barrier layerover the first barrier layer, wherein the second barrier layer is madeof Al_(X)Ga_((1-X))N that is between 10 Å and 100 Å thick.
 20. Themethod of making the HEMT device of claim 11 further including disposinga buffer layer made of GaN over the substrate before disposing thechannel layer.